Optical pickup device

ABSTRACT

An astigmatism element converges laser light in a first direction for generating a first focal line, and converges the laser light in a second direction perpendicular to the first direction for generating a second focal line. A light separating element guides the laser light entered into two first areas and into two second areas to four positions different from each other. The light separating element imparts a light separating function to the laser light entered into the first two areas in directions identical to each other and with magnitudes different from each other, and imparts a light separating function to the laser light entered into the two second areas in directions identical to each other and with magnitudes different from each other.

This application claims priority under 35 U.S.C. Section 119 of JapanesePatent Application No. 2011-144942 filed Jun. 29, 2011, entitled“OPTICAL PICKUP DEVICE”. The disclosure of the above application isincorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical pickup device, and moreparticularly to an arrangement for use in irradiating laser light onto arecording medium having plural laminated recording layers.

2. Disclosure of Related Art

In recent years, the number of recording layers has been increasing, asthe data capacity of an optical disc increases. The data capacity of adisc can be remarkably enhanced by forming plural recording layers inthe one disc. In the case where recording layers are laminated,generally, two layers have been formed on one side of a disc. In recentyears, however, a disc having three or more recording layers on one sidethereof has been put into practical use in order to further increase thedata capacity. An increase in the number of laminated recording layersenables to increase the data capacity of a disc. An increase in thenumber of laminated recording layers, however, may narrow the intervalbetween the recording layers, and increase signal degradation resultingfrom an interlayer crosstalk.

Laminating recording layers weakens reflected light from a recordinglayer (a target recording layer) to be recorded/reproduced. Accordingly,incidence of unwanted reflected light (stray light) from a recordinglayer at an upper position or a lower position of the target recordinglayer into a photodetector may degrade a detection signal, and adverselyaffect focus servo control and tracking servo control. In view of this,in the case where a large number of recording layers are laminated, itis necessary to properly remove stray light, and stabilize a signal froma photodetector.

Japanese Unexamined Patent Publication No. 2009-211770 (corresponding toU.S. Patent Application Publication No. US2009/0225645A1) discloses anovel arrangement of an optical pickup device capable of properlyremoving stray light, in the case where a large number of recordinglayers are formed. With this arrangement, it is possible to form arectangular area (signal light area) where only signal light exists, ona light receiving surface of a photodetector. Reflected light from arecording medium is irradiated at positions near vertex angles of thesignal light area. By disposing sensors of a photodetector at thepositions near the vertex angles of the signal light area, it ispossible to suppress an influence on detection signals resulting fromstray light.

In the optical pickup device thus constructed, an angular adjuster forchanging the propagating direction of reflected light from a disc isused for irradiating signal light onto a signal light area, and thesensor is set to such a sufficiently large size as to receive signallight. However, if such an angular adjuster is configured of a step-typediffraction pattern, which is inexpensive, and plus first-orderdiffraction light caused by diffraction is irradiated onto the signallight area, minus first-order diffraction light which is not expected tobe used may be entered into the sensor. Further, in this case, straylight of plus first-order diffraction light and stray light of minusfirst-order diffraction light may overlap and interfere with each other,which may form an interference fringe on the sensor.

SUMMARY OF THE INVENTION

A main aspect of the invention relates to an optical pickup device. Theoptical pickup device according to the main aspect includes a laserlight source; an objective lens which focuses laser light emitted fromthe laser light source on a recording medium; an astigmatism elementinto which the laser light reflected on the recording medium is entered,and which converges the laser light in a first direction for generatinga first focal line and converges the laser light in a second directionperpendicular to the first direction for generating a second focal line;a photodetector which receives the laser light passing through theastigmatism element; and a light separating element into which the laserlight reflected on the recording medium is entered, and which guides thelaser light entered into two first areas and into two second areas tofour positions different from each other, on a light receiving surfaceof the photodetector. In this arrangement, the photodetector has aplurality of sensing portions disposed at the four different positions.When an intersection of two straight lines respectively extending inparallel to the first direction and the second direction andintersecting with each other is aligned with an optical axis of thelaser light, the two first areas are disposed in a direction along whichone pair of vertically opposite angles defined by the two straight linesare aligned, and the two second areas are disposed in a direction alongwhich another pair of vertically opposite angles are aligned. The lightseparating element imparts a light separating function to the laserlight entered into the two first areas in directions identical to eachother and with magnitudes different from each other, and imparts a lightseparating function to the laser light entered into the two second areasin directions identical to each other and with magnitudes different fromeach other.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects, and novel features of the present inventionwill become more apparent upon reading the following detaileddescription of the embodiment along with the accompanying drawings.

FIGS. 1A and 1B are diagrams for describing a technical principle (as tohow laser light converges) in an embodiment of the invention.

FIGS. 2A through 2D are diagrams for describing the technical principle(as to how light flux areas are distributed) in the embodiment.

FIGS. 3A through 3D are diagrams for describing the technical principle(as to how signal light and stray light are distributed) in theembodiment.

FIGS. 4A and 4B are diagrams for describing the technical principle (amethod for extracting only signal light) in the embodiment.

FIGS. 5A and 5B are diagrams for describing the technical principle (themethod for extracting only signal light) in the embodiment.

FIGS. 6A and 6B are diagrams for describing the technical principle (themethod for extracting only signal light) in the embodiment.

FIGS. 7A and 7B are diagrams for describing a sensor and a signalgenerating method based on a conventional astigmatism method.

FIGS. 8A through 8C are diagrams for describing a sensor and a signalgenerating method based on the technical principle in the embodiment.

FIGS. 9A through 9C are diagrams showing an optical system of an opticalpickup device in an example of the invention.

FIGS. 10A through 10C are diagrams showing an arrangement of a lightseparating element in the example.

FIG. 11 is a diagram showing a sensor layout of a photodetector in theexample.

FIG. 12 is a schematic diagram showing irradiation areas of zero-thorder diffraction light, plus first order diffraction light, minus firstorder diffraction light in the example.

FIG. 13A is a plan view of a light separating element and FIG. 13B is adiagram showing a sensor layout of a photodetector as a modification ofthe example.

FIG. 14 is a diagram showing a position adjusting process for theoptical pickup device in the example.

FIGS. 15A through 15C are diagrams for describing an arrangement of alight separating element as a modification of the example.

FIGS. 16A through 16C are diagrams for describing an arrangement of alight separating element as another modification of the example.

FIGS. 17A and 17B are diagrams showing a simulation result onirradiation areas near sensing portions, and FIG. 17C is a diagramshowing a simulation result on the relations between a lens shift amountand a ratio of stray light entered into the sensing portions, based onthe technical principle in the embodiment.

FIGS. 18A through 18F are diagrams showing a simulation result onirradiation areas near the sensing portions based on the technicalprinciple in the embodiment.

FIGS. 19A through 19F are diagram showing a simulation result onirradiation areas near the sensing portions based on the technicalprinciple in the embodiment.

FIGS. 20A and 20B are diagrams showing a simulation result on therelations between a lens shift amount and a ratio of stray light enteredinto the sensing portions based on the technical principle in theembodiment.

FIG. 21A is a plan view showing a light separating element and FIG. 21Bis a diagram showing a portion near a center of a photodetector as amodification of the example.

FIGS. 22A and 22B are plan views showing light separating elements asmodifications of the example.

The drawings are provided mainly for describing the present invention,and do not limit the scope of the present invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

In the following, an embodiment of the invention is described referringto the drawings.

Technical Principle

Firstly, a technical principle to which the embodiment of the inventionis applied is described referring to FIGS. 1A through 8C.

FIGS. 1A, 1B are diagrams for describing as to how laser light isconverged. FIG. 1A is a diagram showing a state as to how laser light(signal light) reflected on a target recording layer, laser light (straylight 1) reflected on a layer located at a rearward position withrespect to the target recording layer, and laser light (stray light 2)reflected on a layer located at a forward position with respect to thetarget recording layer are converged. FIG. 1B is a diagram showing anarrangement of an anamorphic lens to be used in the technical principle.

Referring to FIG. 1B, the anamorphic lens has a function of converginglaser light to be entered in a direction in parallel to the optical axisof the anamorphic lens in a curved surface direction and in a flatsurface direction. In this embodiment, the curved surface direction andthe flat surface direction intersect perpendicularly to each other.Further, the curved surface direction has a smaller radius of curvaturethan that of the flat surface direction, and has a greater effect ofconverging laser light to be entered into the anamorphic lens.

In the present specification, to simplify the description on theastigmatism function of the anamorphic lens, the terms “curved surfacedirection” and “flat surface direction” are used. Actually, however, asfar as the anamorphic lens has a function of forming focal lines on theoptical axis of the anamorphic lens at different positions from eachother, the shape of the anamorphic lens in the “flat surface direction”in FIG. 1B is not limited to a flat plane shape. In the case where laserlight is entered into the anamorphic lens in a convergence state, theshape of the anamorphic lens in the “flat surface direction” may be astraight line shape (where the radius of curvature=∞).

Referring to FIG. 1A, signal light converged by the anamorphic lensforms focal lines at different positions from each other by convergencein the curved surface direction and in the flat surface direction. Thefocal line position (P02) of signal light by convergence in the curvedsurface direction is closer to the anamorphic lens than the focal lineposition (P03) of signal light by convergence in the flat surfacedirection, and the convergence position (P01) of signal light is anintermediate position between the focal line positions (P02) and (P03)of signal light by convergence in the curved surface direction and inthe flat surface direction. The beam spot of signal light has a shape ofa least circle of confusion at the convergence position (P01). A planeperpendicular to the optical axis of laser light to be entered into theanamorphic lens, at the convergence position (P01), is hereinaftercalled as a “plane P0”.

Likewise, the focal line position (P12) of stray light 1 converged bythe anamorphic lens in the curved surface direction is closer to theanamorphic lens than the focal line position (P13) of stray light 1 byconvergence in the flat surface direction. The anamorphic lens isdesigned to set the focal line position (P13) of stray light 1 byconvergence in the flat surface direction closer to the anamorphic lensthan the convergence position (P01) of signal light.

Likewise, the focal line position (P22) of stray light 2 converged bythe anamorphic lens in the curved surface direction is closer to theanamorphic lens than the focal line position (P23) of stray light 2 byconvergence in the flat surface direction. The anamorphic lens isdesigned to set the focal line position (P22) of stray light 2 byconvergence in the curved surface direction away from the anamorphiclens than the convergence position (P01) of signal light.

The following is a description on the relations between light flux areasof signal light and stray light 1, 2 on the plane P0, taking intoaccount the above matter.

FIG. 2A is a diagram showing four light flux areas f1 through f4 definedfor laser light to be entered into the anamorphic lens. In thisarrangement, signal light passing through the light flux areas f1through f4 is distributed on the plane P0, as shown in FIG. 2B. Straylight 1 passing through the light flux areas f1 through f4 isdistributed on the plane P0, as shown in FIG. 2C. Stray light 2 passingthrough the light flux areas f1 through f4 is distributed on the planeP0, as shown in FIG. 2D. In FIGS. 2B through 2D, the circle indicatingthe beam diameter of signal light is indicated by the solid line, and asshown in FIGS. 2C, 2D, stray light 1 and stray light 2 spread with alarger area than the area of signal light.

In the above arrangement, if signal light and stray light 1, 2 on theplane P0 are extracted in each of the light flux areas, the distributionof each light is as shown in FIGS. 3A through 3D. In this arrangement,signal light passing through each light flux area does not overlap straylight 1 and stray light 2 passing through the corresponding light fluxarea. Accordingly, if the device is configured in such a manner thatonly signal light is received by sensors, after signal light and straylight 1, 2 passing through the each light flux area are separated indirections different from each other, only signal light is entered intothe corresponding sensor, which makes it possible to suppress incidenceof stray light. Thus, it is possible to avoid degradation of a detectionsignal resulting from stray light.

As described above, it is possible to extract only signal light bydispersing the light passing through the light flux areas f1 through f4from each other on the plane S0. The embodiment is made based on theabove technical principle.

FIG. 4A is a diagram showing vectors to be imparted to the propagatingdirections of laser light passing through the light flux areas f1through f4 for separating laser light (signal light and stray light 1,2) passing through the light fluxes areas f1 through f4 on the plane P0.FIG. 4A is a diagram of the anamorphic lens when viewed from thepropagating directions of laser light at the time of incidence into theanamorphic lens.

The propagating directions of laser light passing through the light fluxareas f1 through f4 are respectively changed by imparting vectors V01through V04. The directions of the vectors V01 through V04 are inclinedby 45 degrees with respect to the flat surface direction and withrespect to the curved surface direction. The directions of the vectorsV01 and V02 coincide with each other, and the directions of the vectorsV03 and V04 coincide with each other. Further, the magnitudes of thevectors V01 and V04 are equal to each other, and the magnitudes of thevectors V02 and V03 are equal to each other. The magnitudes of thevectors V01 through V04 are defined as angles with respect to thepropagating directions of laser light before these vectors are imparted(the propagating directions of laser light at the time of incidence intothe anamorphic lens).

When the propagating directions are changed as shown in FIG. 4A, laserlight (signal light and stray light 1, 2) passing through the light fluxareas f1 through f4 are irradiated on the plane P0, as shown in FIG. 4B.FIG. 4B also shows a center O representing the optical axis of laserlight before the propagating directions are changed. Adjusting thedirections and the magnitudes of the vectors V01 through V04 makes itpossible to distribute signal light and stray light 1, 2 passing througheach of the light flux areas f1 through f4, on the plane P0, as shown inFIG. 4B. With this arrangement, it is possible to define an area whereonly the irradiation area of signal light exists on the plane P0.

Alternatively, as shown in FIG. 5A, the propagating directions of laserlight passing through the light flux areas f1 through f4 mayberespectively changed by imparting vectors V11 through V14, in place ofthe arrangement shown in FIG. 4A. In the modification, the directions ofthe vectors V11 through V14 coincide with the directions of the vectorsV01 through V04 shown in FIG. 4A. Further, the magnitude of the vectorV12 is larger than the magnitude of the vector V11, and the magnitude ofthe vector V13 is larger than the magnitude of the vector V14.

If the propagating directions are changed as shown in FIG. 5A, laserlight (signal light and stray light 1, 2) passing through the light fluxareas f1 through f4 are irradiated on the plane P0, as shown in FIG. 5B.In this modification, it is also possible to define an area where onlythe irradiation area of signal light exists on the plane P0.

Further alternatively, as shown in FIG. 6A, the propagating directionsof laser light passing through the light flux areas f1 through f4 may berespectively changed by imparting vectors V21 through V24, in place ofthe arrangement shown in FIG. 4A. In this modification, the directionsof the vectors V21 through V24 coincide with the directions of thevectors V01 through V04 shown in FIG. 4A. Further, the magnitude of thevector V21 is larger than the magnitude of the vector V22, and themagnitude of the vector V24 is larger than the magnitude of the vectorV23.

If the propagating directions are changed as shown in FIG. 6A, laserlight (signal light and stray light 1, 2) passing through the light fluxareas f1 through f4 are irradiated on the plane P0, as shown in FIG. 6B.In this modification, it is also possible to define an area where onlythe irradiation area of signal light exists on the plane P0.Specifically, in this modification, the irradiation areas of laser light(signal light) passing through the light flux areas f1, f2 are locatedat vertex angular positions corresponding to diagonal positions of arectangle (signal light area 1) where only these two irradiation areasexist, and the irradiation areas of laser light (signal light) passingthrough the light flux areas f3, f4 are located at vertex angularpositions corresponding to diagonal positions of a rectangle (signallight area 2) where only these two irradiation areas exist.

In the following, a sensor and a signal generating method based on aconventional astigmatism method are described.

FIG. 7A is a diagram showing eight light flux areas a1 through a8defined for reflected light from a disc, and FIG. 7B is a diagramshowing irradiation areas of signal light and a sensor based on aconventional astigmatism method. The sensor shown in FIG. 7B is disposedon the plane P0 in the arrangement shown in FIG. 1A. FIG. 7B showsirradiation areas A1 through A8, on the plane P0, which are respectivelyirradiated with signal light passing through the light flux areas a1through a8.

Referring to FIG. 7A, the direction of a diffraction image (a trackimage) of signal light resulting from a track groove has an inclinationof 45 degrees with respect to the flat surface direction and withrespect to the curved surface direction, and extends in up and downdirections. Accordingly, in FIG. 7B, the direction of a track image ofsignal light extends in left and right directions. In FIGS. 7A, 7B, atrack image is indicated by the dotted line.

Referring to FIG. 7B, in the conventional astigmatism method, afour-divided sensor composed of four sensing portions Sa through Sd isdisposed on a light receiving surface of a photodetector. In thisembodiment, to simplify the description, each of the sensing portions Sathrough Sd is divided into two portions along the flat surface directionor along the curved surface direction. In other words, the sensingportion Sa is divided into sensing portions S1 and S2, the sensingportion Sb is divided into sensing portions S3 and S4, the sensingportion Sc is divided into sensing portions S5 and S6, and the sensingportion Sd is divided into sensing portions S7 and S8. In thisarrangement, assuming that detection signals to be obtained by thesensing portions S1 through S8 are respectively expressed by S1 throughS8, a focus error signal FE and a push-pull signal PP can berespectively acquired by the following equations (1) and (2).FE=(S3+S4+S7+S8)−(S1+S2+S5+S6)  (1)PP=(S1+S2+S3+S4)−(S5+S6+S7+S8)  (2)

Next, the sensor for receiving signal light shown in FIGS. 4B, 5B, 6Band a signal generating method are described.

FIGS. 8A through 8C are diagrams respectively showing the sensingportions for receiving signal light whose propagating directions arechanged, as shown in FIGS. 4A, 5A, 6A. In FIGS. 8A through 8C, thesensing portions S1 through S8 are disposed on the plane P0, and thedirection of a track image extends in left and right directions.

If the propagating directions of signal light are changed as shown inFIG. 4A, signal light passing through the light flux areas a1 through a8shown in FIG. 7A are respectively irradiated onto irradiation areas A1through A8 shown in FIG. 8A. Likewise, if the propagating directions ofsignal light are changed as shown in FIGS. 5A, 6A, signal light passingthrough the light flux areas a1 through a8 are irradiated ontoirradiation areas A1 through A8 shown in FIGS. 8B, 8C.

Accordingly, disposing the sensing portions S1 through S8 at thepositions of the irradiation areas A1 through A8 of signal light, asshown in FIGS. 8A through 8C, enables to acquire a focus error signal FEand a push-pull signal PP by the equations (1) and (2) in the samemanner as in the arrangement shown in FIG. 7B.

As described above, according to the technical principle in theembodiment, it is possible to generate a focus error signal and apush-pull signal (a tracking error signal) in which an influence ofstray light is suppressed by performing the same computation process asapplied to the conventional astigmatism method.

In the embodiment, as shown in FIGS. 8A through 8C, the sensing portionsS1 through S8 are normally set to such a sufficiently large size as toinclude the irradiation areas A1 through A8. Configuring the sensingportions S1 through S8 as described above, however, may causeoverlapping between an irradiation area other than the irradiation areaas a target light receiving area, and the sensing portions S1 through S8in FIG. 8A. Specifically, in FIG. 8A, the irradiation areas A3, A2 mayoverlap a lower end of the sensing portions S6, S7, the irradiationareas A6, A7 may overlap an upper end of the sensing portions S3, S2,the irradiation areas A5, A4 may overlap a left end of the sensingportions S8, S1, and the irradiation area A8, A1 may overlap a right endof the sensing portions S5, S4.

Further, in FIG. 8A, the irradiation areas of stray light 1, 2 as shownin FIG. 4B are distributed in addition to the irradiation area A1through A8 of signal light. In this arrangement, the irradiation area ofstray light 1 and the irradiation area of stray light 2 overlap eachother substantially at the same position adjacent to the sensingportions S1 through S8. In this case, there is a likelihood that aninterference fringe resulting from overlapping between stray light 1, 2maybe formed on the sensing portions S1 through S8.

On the other hand, in the case where the sensing portions S1 through S8shown in FIGS. 8B, 8C receive signal light corresponding to theirradiation areas A1 through A8, with respect to the irradiation areasshown in FIGS. 5B, 6B, unlike the arrangement shown in FIG. 8A, it isless likely that an irradiation area other than the irradiation area asa target light receiving area may overlap the sensing portions S1through S8, and an interference fringe resulting from overlappingbetween stray light 1, 2 is less likely to be formed on the sensingportions S1 through S8.

Specifically, in FIGS. 5B, 6B, the interval between two signal lightarranged in up and down directions, and the interval between two signallight arranged in left and right directions are set to a larger value,as compared with the arrangement shown in FIG. 4B. With thisarrangement, it is less likely that an irradiation area of signal lightother than a target light receiving area may overlap the sensingportions S1 through S8 shown in FIGS. 8B, 8C. Further, as shown in FIGS.5B, 6B, an area where stray light 1,2 overlap each other is small, ascompared with the arrangement shown in FIG. 4B, and is away from signallight. With this arrangement, it is less likely that an interferencefringe resulting from overlapping between stray light 1,2 maybe formedon the sensing portions S1 through S8 shown in FIGS. 8B, 8C.

As described above, in the case where signal light is received on thesensing portions S1 through S8 shown in FIGS. 8B, 8C, it is possible toobtain a detection signal with high precision, as compared with the casewhere signal light is received on the sensing portions S1 through S8shown in FIG. 8A.

The manner as to how light is separated as shown in FIGS. 5A, 6Acorresponds to a manner as to how light is separated according to anembodiment of the invention.

In the following example, there is shown a concrete arrangement exampleof an optical pickup device based on the principle which is applied tothe case where irradiation areas are distributed as shown in FIG. 6B oras shown in FIG. 7B.

EXAMPLE

In the present example, a semiconductor laser 101 corresponds to a“laser light source” in the claims. A two-wavelength laser 103corresponds to “another laser light source” in the claims. A polarizedbeam splitter 106, a collimator lens 108, a quarter wave plate 110, anda rise-up mirror 111 correspond to an “optical system” in the claims. Atwo-wavelength objective lens 113 corresponds to “another objectivelens” in the claims. A BD objective lens 114 corresponds to an“objective lens” in the claims. An anamorphic lens 115 corresponds to an“astigmatism element” in the claims. One of a flat surface direction anda curved surface direction corresponds to a “first direction” in theclaims, and the other of the flat surface direction and the curvedsurface direction corresponds to a “second direction” in the claims.Sensing portions Ba1 to Ba4, Bs1 to Bs4 correspond to a “plurality ofsensing portions” in the claims. A four-divided sensor C1 corresponds toa “four-divided sensor” in the claims. Four-divided sensors C2, C3correspond to “another four-divided sensors” in the claims. Diffractionareas H11, H12 and diffraction areas H21, H22 correspond to “firstareas” in the claims. Diffraction areas H13, H14 and diffraction areasH23, H24 correspond to “second areas” in the claims. A diffraction areaH15 and a diffraction area H25 correspond to a “third area” in theclaims. Signal light area 1 corresponds to a “first rectangle” in theclaims, and signal light area 2 corresponds to a “second rectangle”. Thedescription regarding the correspondence between the claims and thepresent example is merely an example, and the claims are not limited bythe description of the present example.

Further, in modification examples to be described later, diffractionareas H31, H32 and diffraction areas H41, H42 correspond to the “firstareas” in the claims. Diffraction areas H33 to H36 and diffraction areasH43 to H46 correspond to the “second areas” in the claims. A diffractionarea H37 and a diffraction area H47 correspond to the “third area” inthe claims. The description regarding the correspondence between theclaims and the modification examples is merely an example, and theclaims are not limited by the description of the modification examples.

The present example is an example, wherein the invention is applied toan optical pickup device compatible with BD, DVD and CD. Theaforementioned principle is applied only to an optical system for BD,and a focus adjusting technology by a conventional astigmatism methodand a tracking adjusting technology by a 3-beam method (an in-linemethod) are applied to an optical system for CD and an optical systemfor DVD.

FIGS. 9A, 9B are diagrams showing an optical system of an optical pickupdevice in the present example. FIG. 9A is a plan view of the opticalsystem, wherein the arrangement of the optical system on the disc sidewith respect to rise-up mirrors 111, 112 is omitted, and FIG. 9B is aperspective side view of the optical system posterior to the rise-upmirrors 111, 112.

As shown in FIGS. 9A, 9B, the optical pickup device is provided with asemiconductor laser 101, a half wave plate 102, a two-wavelength laser103, a diffraction grating 104, a dichroic mirror 105, a polarized beamsplitter 106, a front monitor 107, a collimator lens 108, a drivingmechanism 109, a quarter wave plate 110, the rise-up mirrors 111, 112, atwo-wavelength objective lens 113, a BD objective lens 114, a lightseparating element H1, an anamorphic lens 115, and a photodetector 116.

The semiconductor laser 101 emits laser light (hereinafter, called as“BD light”) for BD and having a wavelength of or about 405 nm. The halfwave plate 102 adjusts the polarization direction of BD light in such amanner that the polarization direction of BD light is slightly displacedfrom the polarization direction of S-polarized light with respect to thepolarized beam splitter 106. The two-wavelength laser 103 accommodates,in a certain CAN, two laser elements which emit laser light(hereinafter, called as “CD light”) for CD and having a wavelength of orabout 785 nm, and laser light (hereinafter, called as “DVD light”) forDVD and having a wavelength of or about 660 nm. The two-wavelength laser103 is disposed at such a position that the polarization directions ofCD light and DVD light to be emitted from the two-wavelength laser 103are slightly displaced from the polarization direction of S-polarizedlight with respect to the polarized beam splitter 106.

FIG. 9C is a diagram showing an arrangement of the laser elements (laserlight sources) in the two-wavelength laser 103. FIG. 9C is a diagram ofthe two-wavelength laser 103 when viewed from the beam emission side. CDlight and DVD light are respectively emitted from emission points 103 a,103 b, and a predetermined gap is formed between the emission point 103a and the emission point 103 b. As will be described later, the gapbetween the emission point 103 a of CD light and the emission point 103b of DVD light is set to such a value that DVD light is properlyirradiated onto four-divided sensors for DVD light. Accommodating twolight sources in one CAN as described above is advantageous insimplifying the optical system, as compared with an arrangement providedwith plural CANs.

Referring back to FIG. 9A, the diffraction grating 104 is a two-stepdiffraction grating, and separates each of CD light and DVD light into amain beam and two sub beams. The dichroic mirror 105 is internallyformed with a dichroic surface 105 a. The dichroic surface 105 areflects BD light and transmits CD light and DVD light. Thesemiconductor laser 101, the two-wavelength laser 103 and the dichroicmirror 105 are disposed at such positions that the optical axis of BDlight reflected on the dichroic surface 105 a, and the optical axis ofCD light transmitted through the dichroic surface 105 a are aligned witheach other. The optical axis of DVD light transmitted through thedichroic surface 105 a is displaced from the optical axes of BD lightand CD light by the gap shown in FIG. 9C.

A part of each of BD light, CD light and DVD light is transmittedthrough the polarized beam splitter 106, and a main part thereof isreflected on the polarized beam splitter 106. As described above, thehalf wave plate 102, the two-wavelength laser 103 and the diffractiongrating 104 are disposed at such positions that a part of each of BDlight, CD light and DVD light is transmitted through the polarized beamsplitter 106.

When the diffraction grating 104 is disposed at the position asdescribed above, a main beam and two sub beams of CD light, and a mainbeam and two sub beams of DVD light are respectively aligned along thetracks of CD and DVD. The main beam and the two sub beams reflected onCD are irradiated onto four-divided sensors for CD on the photodetector116 to be described later. The main beam and two sub beams reflected onDVD are irradiated onto the four-divided sensors for DVD on thephotodetector 116 to be described later.

BD light, CD light, DVD light transmitted through the polarized beamsplitter 106 is irradiated onto the front monitor 107. The front monitor107 outputs a signal in accordance with a received light amount. Thesignal from the front monitor 107 is used for emission power control ofthe semiconductor laser 101 and the two-wavelength laser 103.

The collimator lens 108 converts BD light, CD light, DVD light to beentered from the side of the polarized beam splitter 106 into parallellight. The driving mechanism 109 moves the collimator lens 108 in theoptical axis direction in accordance with a control signal foraberration correction. The driving mechanism 109 is provided with aholder 109 a for holding the collimator lens 108, and a gear 109 b forfeeding the holder 109 a in the optical axis direction of the collimatorlens 108. The gear 109 b is connected to a driving shaft of a motor 109c.

BD light, CD light, DVD light collimated by the collimator lens 108 isentered into the quarter wave plate 110. The quarter wave plate 110converts BD light, CD light, DVD light to be entered from the side ofthe collimator lens 108 into circularly polarized light, and converts BDlight, CD light, DVD light to be entered from the side of the rise-upmirror 111 into a linearly polarized light whose polarization directionis orthogonal to the polarization direction of BD light, CD light, DVDlight at the time of incidence from the side of the collimator lens 108.By performing the above operation, light reflected on a disc istransmitted through the polarized beam splitter 106. The optical axis ofreflected light from a disc, which is transmitted through the polarizedbeam splitter 106, is aligned with Z axis shown in FIG. 9A.

The rise-up mirror 111 is a dichroic mirror. The rise-up mirror 111transmits BD light, and reflects CD light and DVD light in a directiontoward the two-wavelength objective lens 113. The rise-up mirror 112reflects BD light in a direction toward the BD objective lens 114.

The two-wavelength objective lens 113 is configured to properly focus CDlight and DVD light on CD and DVD, respectively. Further, the BDobjective lens 114 is configured to properly focus BD light on BD. Thetwo-wavelength objective lens 113 and the BD objective lens 114 aredriven by an objective lens actuator 122 in a focus direction and in atracking direction, while being held on a holder 121.

The light separating element H1 distribute laser light passing throughlight flux areas shown in FIG. 6A, on the plane P0, as shown in FIG. 6B.The configuration of the light separating element H1 will be describedlater referring to FIGS. 10A through 10C.

The anamorphic lens 115 corresponds to the anamorphic lens shown in FIG.1A, and introduces astigmatism to BD light, CD light, DVD light to beentered from the side of the light separating element H1. BD light, CDlight, DVD light transmitted through the anamorphic lens 115 is enteredinto the photodetector 116. The photodetector 116 is provided with aplurality of sensors for receiving light. The sensors on thephotodetector 116 will be described later referring to FIG. 11.

FIG. 10A is a plan view of the light separating element H1 when viewedfrom the side of the polarized beam splitter 106. FIG. 10B is a diagramshowing light flux areas a11 through a15 obtained by dividing laserlight to be entered into the light separating element H1 alongborderlines of diffraction areas H11 through H15 of the light separatingelement H1. FIG. 10A also shows the flat surface direction, the curvedsurface direction and the direction of a track image.

The light separating element H1 is made of a square transparent plate,and has a two-step diffraction pattern (a diffraction hologram) on alight incident surface thereof. As shown in FIG. 10A, the light incidentsurface of the light separating element H1 is divided into fivediffraction areas H11 through H15. The diffraction area H15 has such alarge size as to suppress degradation of a detection signal resultingfrom stray light of BD light, and has such a small size as to properlyobtain a tracking error signal TE based on BD light, which will bedescribed later.

The diffraction areas H11 through H15 divide laser light passing throughthe light flux areas a11 through a15 into zero-th order diffractionlight, plus first order diffraction light, minus first order diffractionlight by diffraction. Plus first order diffraction light of laser lightpassing through the light flux areas a11 through a15 is diffracted inthe directions shown by solid line arrows (V21 through V25) in FIG. 10A.Minus first order diffraction light of laser light passing through thelight flux areas a11 through a15 is diffracted in the directions shownby dotted line arrows (V21 m through V25 m) in FIG. 10A. Zero-th orderdiffraction light of laser light passing through the light flux areasa11 through a15 is transmitted through the diffraction areas H11 throughH15 without diffraction.

In FIG. 10A, diffraction directions and magnitudes of diffraction(diffraction angles) to be given to laser light by the diffraction areasH11 through H15 are indicated by the vectors V21 through V25 and thevectors V21 m through V25 m. The propagating directions of plus firstorder diffraction light to be generated by the diffraction areas H11through H15 are respectively obtained by adding the vectors V21 throughV25 to the propagating directions of laser light before incidence intothe diffraction areas H11 through H15. The propagating directions ofminus first order diffraction light to be generated by the diffractionareas H11 through H15 are respectively obtained by adding the vectorsV21 m through V25 m to the propagating directions of laser light beforeincidence into the diffraction areas H11 through H15.

As well as the arrangement shown in FIG. 6A, the directions of thevectors V21 and V22 coincide with each other, and the directions of thevectors V23 and V24 coincide with each other. Further, as well as thearrangement shown in FIG. 6A, the magnitude of the vector V21 is largerthan the magnitude of the vector V22, and the magnitude of the vectorV24 is larger than the magnitude of the vector V23. The vectors V21 mthrough V24 m respectively have directions opposite to those of thevectors V21 through V24 and have magnitudes equal to those of thevectors V21 through V24.

In the present example, the diffraction area H15 is operable to changethe propagating direction of laser light passing through the light fluxarea a15, unlike the arrangement shown in FIG. 6A. The directions of thevectors V25, V25 m to be given by the diffraction area H15 are inparallel to the flat surface direction, and the magnitudes of thevectors V25, V25 m are equal to each other.

The directions of the vectors V21 through V25, V21 m through V25 m aredetermined by the orientation of a diffraction pattern to be set foreach diffraction area, and the magnitudes of the vectors V21 throughV25, V21 m through V25 m are determined by the pitch of a diffractionpattern to be set for each diffraction area.

FIG. 10C is a diagram showing the relations between a step height and adiffraction efficiency of the diffraction areas H11 through H15.

As shown in FIG. 10C, the diffraction efficiencies of BD light, DVDlight, CD light to be entered into the light separating element H1 arechanged by the step height of a two-step diffraction pattern, which isset for the diffraction areas H11 through H15. The step height in thepresent example is set to the “setting value” shown in FIG. 10C. At thesetting value, the diffraction efficiencies of zero-th order diffractionlight and plus first order diffraction light of BD light arerespectively set to about 80% and about 10%, and the diffractionefficiencies of zero-th order diffraction light of DVD light and zero-thorder diffraction light of CD light are set to about 90% or more. Thediffraction efficiency of minus first order diffraction light issubstantially the same as the diffraction efficiency of plus first orderdiffraction light.

As described above, BD light entered into the light separating elementH1 is divided into zero-th order diffraction light, plus first orderdiffraction light, minus first order diffraction light with therespective diffraction efficiencies. Further, a main part of CD lightand DVD light entered into the light separating element H1 istransmitted through the light separating element H1 without beingdiffracted by the light separating element H1.

FIG. 11 is a diagram showing a sensor layout of the photodetector 116.

The photodetector 116 has BD sensing portions Ba1 through Ba4, Bs1through Bs4 for receiving plus first order diffraction light of BD light(signal light) to be generated by the diffraction function of thediffraction areas H11 through H14; a four-divided sensor Bz forreceiving plus first order diffraction light of BD light (signal lightand stray light 1, 2) to be generated by the diffraction function of thediffraction area H15; four-divided sensors C1 through C3 for receivingCD light transmitted through the light separating element H1 withoutbeing diffracted by the light separating element H1; and four-dividedsensors D1 through D3 for receiving DVD light transmitted through thelight separating element H1 without being diffracted by the lightseparating element H1. The sensing portions Ba1 through Ba4, Bs1 throughBs4 are respectively disposed at the same positions as the sensingportions S1 through S8 shown in FIG. 8C, which have been described forexplaining the aforementioned principle. The four-divided sensor C1 isalso used for receiving zero-th order diffraction light of BD light tobe described later.

A center O of the photodetector 116 is an intersection at which theoptical axis of BD light to be emitted from the polarized beam splitter106 in plus Z-axis direction intersects a light receiving surface of thephotodetector 116.

Plus first order diffraction light of BD light (signal light) passingthrough the light flux areas a11 through a15 is irradiated ontoirradiation areas A11 through A15. Light corresponding to theirradiation area A11 is received by the sensing portions Ba1, Ba4, lightcorresponding to the irradiation area A12 is received by the sensingportions Ba2, Ba3, light corresponding to the irradiation area A13 isreceived by the sensing portions Bs3, Bs4, and light corresponding tothe irradiation area A14 is received by the sensing portions Bs1, Bs2.

Plus first order diffraction light of BD light (signal light and straylight 1, 2) passing through the light flux area a15 is entered into thefour-divided sensor Bz located at an upper right position with respectto the center O. The four-divided sensor Bz is composed of sensingportions Bz1 through Bz4, and is used for adjusting the position of thelight separating element H1. The four-divided sensor Bz is disposed withan inclination of 45 degrees with respect to up and down directions andwith respect to left and right directions. Further, the four-dividedsensor Bz is disposed at such a position that a parting line of thefour-divided sensor Bz is aligned with a straight line shown by theone-dotted chain line in FIG. 11, which connects between the center Oand a center Bz0 of the four-divided sensor Bz. The position adjustmentof the light separating element H1 will be described later referring toFIG. 14.

As shown in FIG. 11, the pitch of the diffraction areas H11 through H14is set in such a manner that the irradiation areas A11 through A14 arelocated at the sensing portions Ba1 through Ba4, Bs1 through Bs4.Further, the pitch of the diffraction area H15 is set in such a mannerthat plus first order diffraction light of BD light (signal light andstray light 1, 2) passing through the light flux area a15 is located atthe center Bz0 of the four-divided sensor Bz.

Since the optical axes of BD light and CD light are aligned by thedichroic surface 105 a as described above, a main beam (zero-th orderdiffraction light) of CD light generated by the diffraction grating 104,and zero-th order diffraction light of BD light are irradiated at thecenter O shown in FIG. 11. The four-divided sensor C1 is disposed at thecenter O. The four-divided sensors C2, C3 are disposed in the directionof a track image of CD with respect to a main beam of CD light, on thelight receiving surface of the photodetector 116, to receive sub beamsof CD light. The four-divided sensors C1, C2 and C3 are respectivelycomposed of sensing portions C11 through C14, sensing portions C21through C24 and the sensing portions C31 through C34.

In the present example, the four-divided sensors C2, C3 are disposed atsuch positions that a parting line of the four-divided sensor C2 and aparting line of the four-divided sensor C3 are located on a straightline extending in up and down directions and passing the center O forimplementing a tracking adjusting technology by an in-line method.

Since the optical axis of DVD light is displaced from the optical axisof CD light as described above, a main beam and two sub beams of DVDlight are irradiated at positions displaced from the irradiationpositions of a main beam and two sub beams of CD light, on the lightreceiving surface of the photodetector 116. The four-divided sensors D1through D3 are disposed at the irradiation positions of a main beam andtwo sub beams of DVD light. The distance between a main beam of CD lightand a main beam of DVD light is determined by the gap between theemission point 103 a of CD light and the emission point 103 b of DVDlight shown in FIG. 9C.

FIG. 12 is a schematic diagram showing irradiation areas of zero-thorder diffraction light, plus first order diffraction light, minus firstorder diffraction light of BD light (signal light and stray light 1, 2)distributed on a plane (plane P0) flush with the light receiving surfaceof the photodetector 116. The broken line indicates plus first orderdiffraction light of BD light, the long-chain line indicates zero-thorder diffraction light of BD light, the dotted line indicates minusfirst order diffraction light of BD light. FIG. 12 also shows thesensors shown in FIG. 11.

Forming a two-step diffraction pattern on the diffraction areas H11through H15 of the light separating element H1, as described in thepresent example, allows to distribute the irradiation areas of plusfirst order diffraction light and minus first order diffraction light ofBD light (signal light and stray light 1, 2) symmetrically to each otherwith respect to the center O, and allows to distribute the irradiationarea of zero-th order diffraction light at the center O. In the presentexample, regarding BD light (signal light and stray light 1, 2), onlyzero-th order diffraction light and plus first order diffraction lightare used, and minus first order diffraction light is not used.

Further, since a central part of BD light to be entered into the lightseparating element H1 is irradiated near the four-divided sensor Bz,which is away from the center O, it is less likely that the irradiationareas of plus first order diffraction light of stray light (stray light1, 2) of BD light distributed near the sensing portions Ba1 through Ba4,Bs1 through Bs4 may overlap the sensing portions Ba1 through Ba4, Bs1through Bs4. Specifically, the irradiation areas of stray light 1, 2distributed near an upper end of the sensing portions Ba1, Ba4respectively have such shapes that a left end of the irradiation area ofstray light 1 and a right end of the irradiation area of stray light 2are removed by the diffraction area H15. Likewise, the irradiation areasof stray light 1, 2 distributed near a lower end of the sensing portionsBat, Ba3, near a right end of the sensing portions Bs1, Bs2, near a leftend of the sensing portions Bs3, Bs4 each has such a shape that an endthereof is removed by the diffraction area H15. With this arrangement,even if the BD objective lens 114 is moved in a radial direction of BD,and the optical axis of the BD objective lens 114 is shifted from theoptical axis of laser light, it is less likely that plus first orderdiffraction light of BD light (stray light 1, 2) may be entered into thesensing portions Ba1 through Ba4, Bs1 through Bs4. Further, even if thepositions of the sensing portions Ba1 through Ba4, Bs1 through Bs4 aredisplaced on the light receiving surface of the photodetector 116, it isless likely that plus first order diffraction light of BD light (straylight 1, 2) maybe entered into the sensing portions Ba1 through Ba4, Bs1through Bs4.

In the following, a signal generating method in the present example isdescribed.

As shown in FIG. 11, the irradiation areas A11 through A14 of plus firstorder diffraction light of BD light (signal light) are located on thesensing portions Ba1 through Ba4, Bs1 through Bs4. In the presentexample, a tracking error signal TE for BD is generated, based ondetection signals from these sensing portions. Assuming that detectionsignals from the sensing portions Ba1 through Ba4, Bs1 through Bs4 arerespectively represented as Ba1 through Ba4, Bs1 through Bs4, thetracking error signal TE in the present example can be acquired by thefollowing equation (3).TE={(Ba1+Ba4)−(Ba2+Ba3)}−k×(Bs1+Bs4)−(Bs2+Bs3)}  (3)

In this example, the multiplier k is used, unlike the computation of thepush-pull signal PP expressed by the equation (2). The computationapproach of a tracking error signal TE using the multiplier k isdisclosed in Japanese Unexamined Patent Publication No. 2010-102813(corresponding to U.S. Patent Application Publication No. US2010/0080106A1) filed by the applicant of the present application, and thedisclosure of U.S. Patent Application Publication No. 2010/0080106 A1 isincorporated by reference herein. The tracking error signal TE may beacquired by using the computation approach by the equation (2).

Further, as shown in FIG. 12, the irradiation areas of zero-th orderdiffraction light of BD light (signal light and stray light 1, 2) arelocated on the four-divided sensor C1. In the present example, a focuserror signal FE and an RF signal for BD are generated, based ondetection signals from the sensing portions C11 through C14 (see FIG.11) of the four-divided sensor C1. Assuming that detections signals fromthe sensing portions C11 through C14 are respectively expressed as C11through C14, the focus error signal FE in the present example can beacquired by the following equation (4) in the same manner as acquiringthe focus error signal FE by the equation (1). Further, the RF signal inthe present example can be acquired by the following equation (5).FE=(C11+C13)−(C12+C14)  (4)RF=(C11+C12+C13+C14)  (5)

Zero-th order diffraction light of BD light to be entered into thefour-divided sensor C1 not only includes signal light but also includesstray light 1, 2. However, since the ratio of stray light to zero-thorder diffraction light of BD light to be entered into the four-dividedsensor C1 is about 1/10, there is no or less likelihood that stray lightmay seriously affect acquisition of a focus error signal FE and an RFsignal.

A focus error signal, a tracking error signal and an RF signal for CDare generated based on detection signals from the four-divided sensorsC1 through C3, and a focus error signal, a tracking error signal and anRF signal for DVD are generated based on detection signals from thefour-divided sensors D1 through D3. The focus error signals and thetracking error signals for CD and DVD are generated by using acomputation process by a conventional astigmatism method and acomputation process by a 3-beam method (an in-line method).

As described above, in the present example, only plus first orderdiffraction light of BD light (signal light) is irradiated onto thesensing portions Ba1 through Ba4, Bs1 through Bs4. With the abovearrangement, it is possible to acquire various detection signals (e.g. atracking error signal TE) with high precision while suppressingdegradation of detection signals resulting from stray light.

Further, the central part of BD light to be entered into the lightseparating element H1 is irradiated near the four-divided sensor Bz,which is away from the center O, by the diffraction area H15.Accordingly, it is less likely that the irradiation areas of plus firstorder diffraction light of stray light (stray light 1, 2) of BD lightdistributed near the sensing portions Ba1 through Ba4, Bs1 through Bs4may overlap the sensing portions Ba1 through Ba4, Bs1 through Bs4. Thisis further advantageous in acquiring various detection signals with highprecision.

Furthermore, in the present example, the light separating element H1having a two-step diffraction pattern is used to distribute theirradiation areas of BD light, as shown in FIG. 6B. Forming the two-stepdiffraction pattern as described above results in widely distributingthe irradiation areas as shown in FIG. 12. In the present example,however, there is no need of disposing sensors on a photodetector atsuch positions as to include all the irradiation areas. Specifically, inthe present example, the sensors on the photodetector 116 for receivingBD light are disposed at such positions as to include only theirradiation areas of signal light (zero-th order diffraction light)distributed at the center O, signal light (plus first order diffractionlight) distributed on the upper side and the right side of the center O,and signal light (plus first order diffraction light) distributed on theupper right portion of the photodetector 116. With this arrangement, itis possible to miniaturize the photodetector 116, even with use of theinexpensive two-step light separating element H1, as described in thepresent example.

Alternatively, it is possible to use a light separating element having ablazed diffraction pattern for distributing the irradiation areas asshown in FIG. 6B. The light separating element having a blazeddiffraction pattern, however, is expensive, as compared with the lightseparating element H1 having a two-step diffraction pattern as employedin the present example. In the present example, use of the lightseparating element H1 having an inexpensive two-step diffraction patternis advantageous in suppressing the cost required for the optical pickupdevice.

Further, in the present example, since the zero-th order diffractionlight of BD light (signal light and stray light 1, 2) is entered intothe center O of the photodetector 116, it is possible to acquire a focuserror signal FE and an RF signal for BD by the four-divided sensor C1for CD. Specifically, it is possible to use a part of the four-dividedsensors C1 through C3 for CD for acquiring a focus error signal FE andan RF signal for BD. With this arrangement, it is possible to suppressthe cost required for the optical pickup device without the need ofproviding an additional sensor, and to miniaturize the photodetector.

In the present example, the light separating element H1 and the sensorson the photodetector 116 are configured, based on the arrangement thatthe sensing portions S1 through S8 are disposed as shown in FIG. 8C withrespect to the irradiation areas shown in FIG. 6B. Alternatively, thelight separating element H1 and the sensors on the photodetector 116 maybe configured, based on an arrangement that the sensing portions S1through S8 are disposed as shown in FIG. 8B with respect to irradiationareas shown in FIG. 5B.

FIG. 13A is a plan view showing a light separating element H2 as theabove modification example.

Diffraction areas H21 through H25 of the light separating element H2 areconfigured in such a manner that vectors V11 through V15 are imparted toplus first order diffraction light, and vectors V11 m through V15 m areimparted to minus first order diffraction light. As well as thearrangement shown in FIG. 5A, the directions of the vectors V11 and V12coincide with each other, and the directions of the vectors V13 and V14coincide with each other. Further, as well as the arrangement shown inFIG. 5A, the magnitude of the vector V12 is larger than the magnitude ofthe vector V11, and the magnitude of the vector V13 is larger than themagnitude of the vector V14. The vectors V11 m through V14 mrespectively have directions opposite to those of the vectors V11through V14 and have magnitudes equal to those of the vectors V11through V14. The vectors V15 and V15 m are respectively the same as thevectors V25 and V25 m shown in FIG. 10A.

FIG. 13B is a diagram showing a sensor layout of the photodetector 116in the case where the light separating element H2 is used.

In the above arrangement, the sensing portions Ba1, Ba4 shown in FIG. 11are disposed at a lower side of the sensing portions Bat, Ba3, and thesensing portions Bs1, Bs2 shown in FIG. 11 are disposed at a left sideof the sensing portions Bs3, Bs4 in the same manner as the positions ofthe sensing portions S1 through S8 shown in FIG. 8B. Plus first orderdiffraction light of BD light (signal light) to be entered into thediffraction areas H21 through H25 is irradiated onto irradiation areasA21 through A25. With this arrangement, it is possible to receive onlyplus first order diffraction light of BD light (signal light) in themanner distributed as shown in FIG. 5B.

Position Adjusting Method

In the foregoing example, it is necessary to adjust the positions of thelight separating element H1 and the photodetector 116 in the opticalpickup device in such a manner that plus first order diffraction lightof BD light (signal light) passing through the light flux areas a11through a14 shown in FIG. 10B is properly entered into the sensingportions Ba1 through Ba4, Bs1 through Bs4 shown in FIG. 11. The aboveadjustment can be performed by the following method.

FIG. 14 is a diagram showing a position adjusting process for theoptical pickup device in the present example. The position adjustment iscarried out at the time of assembling the optical pickup device.

In the position adjusting process, firstly, the optical elements otherthan the light separating element H1 and the photodetector 116 aremounted in the optical pickup device (S11). Then, the light separatingelement H1 held on a holder is mounted in the optical pickup device(S12). Then, the photodetector 116 loaded with the sensors shown in FIG.11 on the light receiving surface thereof is mounted in the opticalpickup device (S13). When the above operation is performed, an arm forposition adjustment is connected to the photodetector 116 so thatposition adjustment to be described later can be automatically carriedout.

Then, electric power is supplied to the optical pickup device (S14). Bysupply of the electric power, the semiconductor laser 101 is turned onto emit light, and a disc (e.g. an ROM having one recording layer)loaded for position adjustment is rotated, and BD light is irradiatedonto the disc. Then, the objective lens actuator 122 is driven in theabove state, and the collimator lens 108 is positioned at apredetermined position.

Then, a position adjustment (XY-adjustment) of the photodetector 116 iscarried out in a plane (XY plane shown in FIG. 9A) perpendicular to theoptical axis of BD light to be emitted from the side of the polarizedbeam splitter 106 in plus Z-axis direction. The XY-adjustment of thephotodetector 116 is carried out based on detection signals from thesensing portions C11 through C14 for CD, which receive zero-th orderdiffraction light of BD light. Specifically, assuming that displacementamounts in X-axis direction and in Y-axis direction of the photodetector116 are represented as PDx, PDy, PDx, PDy can be acquired by thefollowing equations (6), (7).PDx={(C13+C14)−(C11+C12)}/(C11+C12+C13+C14)  (6)PDy={(C12+C13)−(C11+C14)}/(C11+C12+C13+C14)  (7)

Then, the position of the photodetector 116 is roughly adjusted in sucha range as to be adjustable by PDx, PDy expressed by the equations (6),(7) (S15). Then, automatic XY-adjustment control for the photodetector116 is turned on in such a manner that the values of PDx, PDy expressedby the equations (6), (7) are set to zero (S16). By performing the aboveoperation, the arm connected to the photodetector 116 is operable tomove the photodetector 116 in XY plane so that the optical axis ofzero-th order diffraction light of BD light coincides with the center Oof the photodetector 116.

Then, focus servo control is turned on (S17), and the BD objective lens114 is moved in Y-axis direction (a direction perpendicular to a disc)shown in FIG. 9B by the objective lens actuator 122 in such a mannerthat the value of the focus error signal FE expressed by the equation(4) is set to zero.

Then, a position adjustment (Z-adjustment) of the photodetector 116 inZ-axis direction is carried out (S18). In the Z-adjustment of thephotodetector 116, firstly, the BD objective lens 114 is moved in aradial direction of a disc in such a manner that the tracking errorsignal TE expressed by the equation (3) is set to zero. Then, the BDobjective lens 114 is moved in a direction perpendicular to the disc,while referring to the RF signal expressed by the equation (5). When theabove operation is performed, the focal point of BD light (signal lightand stray light 1, 2) entered into the four-divided sensor C1 is changedas the BD objective lens 114 is moved, and the amplitude of the RFsignal is changed as the focal point is changed. The position of thephotodetector 116 in Z-axis direction is determined in such a mannerthat the amplitude of the RF signal has a predetermined magnitude.

Then, a position adjustment (XY-adjustment) of the light separatingelement H1 in XY plane is carried out (S19). The XY-adjustment of thelight separating element H1 is carried out based on detection signalsfrom the sensing portions Ba1 through Ba4, Bs1 through Bs4 shown in FIG.11. Specifically, assuming that displacement amounts of the lightseparating element H1 in X-axis direction and in Y-axis direction arerespectively represented as HOEx, HOEy, HOEx, HOEy can be acquired bythe following equations (8), (9).HOEx={(Bs3+Bs4)−(Bs1+Bs2)}/(Bs1+Bs2+Bs3+Bs4)  (8)HOEy={(Ba2+Ba3)−(Ba1+Ba4)}/(Ba1+Ba2+Ba3+Ba4)  (9)

The light separating element H1 is positioned in such a manner that thevalues of HOEx, HOEy expressed by the equations (8), (9) are set to zeroin XY-plane.

Then, a position adjustment (Z-adjustment) of the light separatingelement H1 in Z-axis direction, and a position adjustment (θ-adjustment)of the light separating element H1 in a rotating direction with respectto the center O are carried out (S20). The Z-adjustment and theθ-adjustment of the light separating element H1 are carried out based ondetection signals from the four-divided sensor Bz (sensing portions Bz1through Bz4) for receiving plus first order diffraction light of BDlight. Specifically, assuming that detection signals from the sensingportions Bz1 through Bz4 are respectively represented as Bz1 throughBz4, and a displacement amount of the light separating element H1 inZ-axis direction and a displacement amount of the light separatingelement H1 in a rotating direction with respect to the center O arerespectively expressed as HOEz, HOEθ, HOEz, HOEθ can be acquired by thefollowing equations (10), (11).HOEz={(Bz1+Bz4)−(Bz2+Bz3)}/(Bz1+Bz2+Bz3+Bz4)  (10)HOEθ={(Bz1+Bz2)−(Bz3+Bz4)}/(Bz1+Bz2+Bz3+Bz4)  (11)

The light separating element H1 is positioned in such a manner that thevalue of HOEz expressed by the equation (10) is set to zero, and thatthe value of HOEθ expressed by the equation (11) is set to zero in arotating direction with respect to the center O.

After the positions of the photodetector 116 and the light separatingelement H1 are adjusted in XY plane and in Z-axis direction as describedabove, the light separating element H1 and the photodetector 116 areadhesively mounted in the optical pickup device (S21). In this example,an adhesive resin is coated on a portion of the light separating elementH1 and the photodetector 116 to be adhered to each other, andultraviolet light is irradiated onto the coated adhesive resin foradhering the light separating element H1 and the photodetector 116 toeach other. Then, the automatic XY-adjustment control for thephotodetector 116 is turned off (S22), and the arm for XY-adjustment,which is connected to the photodetector 116, is detached (chucking off)(S23).

As described above, plus first order diffraction light of BD light(signal light) passing through the light flux areas a11 through a14shown in FIG. 10B is allowed to be properly entered into the sensingportions Ba1 through Ba4, Bs1 through Bs4 shown in FIG. 11. The opticalaxes of zero-th order diffraction light of BD light and zero-th orderdiffraction light of CD light to be entered into the photodetector 116coincide with each other, and the four-divided sensors C1 through C3 forCD and the four-divided sensors D1 through D3 for DVD are disposed inadvance on the light receiving surface of the photodetector 116. Withthis arrangement, performing the position adjustments of the lightseparating element H1 and the photodetector 116 based on BD light allowsCD light and DVD light to be properly entered into the four-dividedsensors C1 through C3, D1 through D3, as well as BD light.

Modification Examples

In the foregoing example, the light separating element H1 is used fordistributing the irradiation area of BD light as shown in FIG. 6B.Alternatively, a light separating element H3 shown in FIG. 15A may beused, in place of the light separating element H1 in the foregoingexample.

FIG. 15A is a plan view of the light separating element H3 when viewedfrom the side of the polarized beam splitter 106. FIG. 15B is a diagramshowing light flux areas a31 through a37 obtained by dividing laserlight to be entered into the light separating element H3 alongborderlines of diffraction areas H31 through H37 of the light separatingelement H3.

The light separating element H3 is made of a square transparent plate,and has a two-step diffraction pattern on a light incident surfacethereof, as well as the light separating element H1. As shown in FIG.15A, the light incident surface of the light separating element H3 isdivided into the seven diffraction areas H31 through H37. Thediffraction areas H33, H34, the diffraction areas H35, H36 respectivelyhave such shapes that each of the diffraction areas H13, H14 of thelight separating element H1 shown in FIG. 10A is divided into left andright portions along a straight line extending in up and down directionsand passing a center of the light separating element H3. The diffractionefficiencies and the pitches of the diffraction areas H31 through H37are defined in the same manner as the diffraction efficiencies and thepitches of the corresponding diffraction areas of the light separatingelement H1.

The diffraction areas H31, H32, H37 impart vectors to the propagatingdirections of laser light passing through the light flux areas a31, a32,a37 in the same manner as the light separating element H1. Thediffraction areas H33 through H36 respectively impart vectors V31through V34, vectors V31 m through V34 m to the propagating directionsof laser light passing through the light flux areas a33 through a36. Thevectors V31 through V34 are vectors to be imparted to plus first orderdiffraction light, and the vectors V31 m through V34 m are vectors to beimparted to minus first order diffraction light. The vectors V31 and V32are respectively vectors obtained by adding a downward vector componentand an upward vector component to the vector V23 shown in FIG. 10A, andthe vectors V33 and V34 are respectively vectors obtained by adding adownward vector component and an upward vector component to the vectorV24 shown in FIG. 10A. The vectors V31 m through V34 m respectively havedirections opposite to those of the vectors V31 through V34 and havemagnitudes equal to those of the vectors V31 through V34.

FIG. 15C is a schematic diagram showing irradiation areas of plus firstorder diffraction light of BD light (signal light) located on thesensing portions Bs1 through Bs4 shown in FIG. 11. Since the irradiationareas on sensing portions other than the sensing portions Bs1 throughBs4 are substantially the same as those in the arrangement shown in FIG.12, the description thereof is omitted herein.

As shown in FIG. 15C, plus first order diffraction light of BD light(signal light) passing through the light flux areas a33 through a36 isirradiated onto the irradiation areas A33 through A36. When the aboveoperation is performed, the irradiation areas A33, A34 do not overlapthe borderline between the sensing portion Bs3 and the sensing portionBs4, and the irradiation areas A35, A36 do not overlap the borderlinebetween the sensing portion Bs1 and the sensing portion Bs2.Specifically, allowing the vectors V31 through V34 to have a downwardvector component or an upward vector component as described aboveenables to form a clearance between the irradiation areas A33 and A34,and enables to form a clearance between the irradiation areas A35 andA36. With this arrangement, it is possible to suppress degradation inprecision of detection signals from the sensing portions Bs1 throughBs4, as compared with the arrangement of the light separating elementH1, even in the case where the positions of the sensing portions Bs1through Bs4 are displaced in up and down directions resulting from e.g.aging deterioration.

Regarding the light separating element H2 which has been described as amodification of the foregoing example referring to FIG. 13A, it is alsopossible to divide each of the upper diffraction area H23 and the lowerdiffraction area H24 into left and right portions, as shown in anothermodification example as described below.

FIG. 16A is a plan view showing a light separating element H4 as anothermodification example.

Borderlines of diffraction areas H41 through H47 of the light separatingelement H4 are defined in the same manner as the diffraction areas H31through H37 shown in FIG. 15A. Laser light passing through light fluxareas a41 through a47 shown in FIG. 16B is respectively entered into thediffraction areas H41 through H47.

The diffraction areas H41, H42, H47 impart vectors to the propagatingdirections of laser light passing through the light flux areas a41, a42,a47 in the same manner as the light separating element H2. Thediffraction areas H43 through H46 respectively impart vectors V41through V44, vectors V41 m through 44 m to the propagating directions oflaser light passing through the light flux areas a43 through a46. Thevectors V41 through V44 are vectors to be imparted to plus first orderdiffraction light, and the vectors V41 m through V44 m are vectors to beimparted to minus first order diffraction light. The vectors V41, V42are respectively vectors obtained by adding a downward vector componentand an upward vector component to the vector V13 shown in FIG. 13A, andthe vectors V43, V44 are respectively vectors obtained by adding adownward vector component and an upward vector component to the vectorV14 shown in FIG. 13A. The vectors V41 m through V44 m respectively havedirections opposite to those of the vectors V41 through V44 and havemagnitudes equal to those of the vectors V41 through V44.

FIG. 16C is a schematic diagram showing irradiation areas of plus firstorder diffraction light of BD light (signal light) located on thesensing portions Bs1 through Bs4 shown in FIG. 13B.

As shown in FIG. 16C, plus first order diffraction light of BD light(signal light) passing through the light flux areas a43 through a46 isirradiated onto the irradiation areas A43 through A46. When the aboveoperation is performed, there is formed a clearance between theirradiation areas A43 and A44, and a clearance between the irradiationareas A45 and A46 in the same manner as in the arrangement shown in FIG.15C. With this arrangement, it is possible to suppress degradation inprecision of detection signals from the sensing portions Bs1 throughBs4, as compared with the arrangement of the light separating elementH2, even in the case where the positions of the sensing portions Bs1through Bs4 are displaced in up and down directions resulting from e.g.aging deterioration.

Simulation of Stray Light at the Time of Lens Shift

The inventor of the present application conducted a simulation on aninfluence of stray light on the sensors disposed at the positions asshown in FIGS. 8A through 8C, in the case where plus first orderdiffraction light of BD light (signal light) is received on the sensors.

In the present simulation, there are proposed the following three lightseparating elements Hs1 through Hs3.

The light separating element Hs1 is a light separating element, whereinthe vectors V01 through V04 shown in FIG. 4A are applied to thediffraction areas H31 through H36 shown in FIG. 15A. In thisarrangement, the vectors in the diffraction areas H31, H32 respectivelycorrespond to the vectors V01, V02 shown in FIG. 4A. Further, thevectors in the diffraction areas H33, H34 respectively correspond tovectors obtained by adding a downward vector component and an upwardvector component to the vector V03 shown in FIG. 4A, and the vectors inthe diffraction areas H35, H36 respectively correspond to vectorsobtained by adding a downward vector component and an upward vectorcomponent to the vector V04 shown in FIG. 4A.

The light separating element Hs2 is a light separating element, whereinthe vectors V11 through V14 shown in FIG. 5A are applied to thediffraction areas H31 through H36 shown in FIG. 15A. In thisarrangement, the vectors in the diffraction areas H31, H32 respectivelycorrespond to the vectors V11, V12 shown in FIG. 5A. Further, thevectors in the diffraction areas H33, H34 respectively correspond tovectors obtained by adding a downward vector component and an upwardvector component to the vector V13 shown in FIG. 5A, and the vectors inthe diffraction areas H35, H36 respectively correspond to vectorsobtained by adding a downward vector component and an upward vectorcomponent to the vector V14 shown in FIG. 5A.

The light separating element Hs3 is configured in the same manner as thelight separating element H3 shown in FIG. 15A. Specifically, the lightseparating element Hs3 is a light separating element, wherein thevectors V21 through V24 shown in FIG. 6A are applied to the diffractionareas H31 through H36 shown in FIG. 15A. In this arrangement, thevectors in the diffraction areas H31, H32 respectively correspond to thevectors V21, V22 shown in FIG. 6A. Further, the vectors in thediffraction areas H33, H34 respectively correspond to vectors obtainedby adding a downward vector component and an upward vector component tothe vector V23 shown in FIG. 6A, and the vectors in the diffractionareas H35, H36 respectively correspond to vectors obtained by adding adownward vector component and an upward vector component to the vectorV24 shown in FIG. 6A.

Further, in the case where the light separating element Hs1 is used, thesensor shown in FIG. 8A is prepared, in the case where the lightseparating element Hs2 is used, the sensor shown in FIG. 8B is prepared,and in the case where the light separating element Hs3 is used, thesensor shown in FIG. 8C is prepared.

In any of the cases where the light separating elements Hs1 through Hs3are used, as shown in FIG. 15C, the irradiation areas of signal light onthe sensing portions located on the right side of the center O do notoverlap the borderline between the sensing portions arranged in up anddown directions.

Further, in the present simulation, BD has four recording layers, andthe recording layers are arranged in the order of L3, L2, L1, L0 fromthe surface side (the light incident surface side) of BD. Further, aphenomenon that a BD objective lens (corresponding to the BD objectivelens 114 in the foregoing example) is moved in a radial direction of BD,and the optical axis of the BD objective lens is shifted with respect tothe optical axis of laser light is hereinafter called as a “lens shift”.

FIGS. 17A, 17B are diagrams showing a simulation result in the casewhere the light separating element Hs1 is used. FIGS. 17A, 17Brespectively show distribution states of signal light and stray lightnear the upper-side sensing portions and the right-side sensing portionswith respect to the center O. In this simulation, BD light is focused onthe recording layer L2, and there is no lens shift. In FIGS. 17A, 17B,the reference sign “L2” denotes reflected light (signal light) from therecording layer L2, and the reference sign “L3” denotes reflected light(stray light) from the recording layer L3.

In the case where there is no les shift, as shown in FIG. 17A, signallight is properly irradiated onto the upper-side sensing portions, withno or less irradiation of stray light onto the upper-side sensingportions. On the other hand, as shown in FIG. 17B, although signal lightis properly irradiated onto the right-side sensing portions, stray lightis irradiated onto the right-side sensing portions with a large area, ascompared with the state shown in FIG. 17A.

In this simulation, if there is a lens shift from the states shown inFIGS. 17A, 17B, stray light is shifted in left and right directions andis entered into both of the upper-side sensing portions and theright-side sensing portions with a large area. For instance, if straylight is shifted in a left direction, in the case of FIG. 17A, theright-side stray light from the recording layer L3 is entered into bothof the upper right sensing portion and the lower right sensing portion.Further, in the case of FIG. 17B, although the lower-side stray lightfrom the recording layer L3 is entered only into the lower left sensingportion, the upper-side stray light from the recording layer L3 isentered into both of the upper left sensing portion and the upper rightsensing portion.

FIG. 17C is a diagram showing a simulation result on the relationsbetween a lens shift amount and a ratio of stray light entered intosensing portions, in the case where the light separating element Hs1 isused. In FIG. 17C, the horizontal axis denotes a lens shift amount ofthe BD objective lens, and the vertical axis denotes a ratio of straylight to a total amount of light to be entered into the eight sensingportions shown in FIGS. 17A, 17B. FIG. 17C clearly shows that a largeamount of stray light is entered into the sensing portions depending onthe lens shift amount, in the case where the light separating elementHs1 is used, and detection signals from the sensing portions aredegraded.

FIGS. 18A through 18F are diagrams showing a simulation result in thecase where the light separating element Hs2 is used. FIGS. 18A, 18C and18E show distribution states of signal light and stray light near theupper-side sensing portions with respect to the center O. FIGS. 18B, 18Dand 18F show distribution states of signal light and stray light nearthe right-side sensing portions with respect to the center O. In thissimulation, BD light is focused on the recording layer L2. In FIGS. 18Athrough 18F, the reference sign “L2” denotes reflected light (signallight) from the recording layer L2, the reference signs “L1”, “L3”respectively denote reflected light (stray light) from the recordinglayers L1, L3, and the term “surface” denotes reflected light from thedisc surface (light incident surface).

FIGS. 18A, 18B show cases that there is no les shift, FIGS. 18C, 18Dshow cases that stray light is shifted in a left direction resultingfrom a lens shift, and FIGS. 18E, 18F show cases that stray light isshifted in a right direction resulting from a lens shift.

As shown in FIGS. 18A through 18F, signal light is properly irradiatedonto the sensing portions regardless of presence of absence of a lensshift.

In the case where there is no lens shift, as shown in FIG. 18A, theirradiation area of stray light from the disc surface overlaps both ofthe lower left sensing portion and the lower right sensing portion ofthe upper-side sensing portions. However, since the irradiation area ofstray light from the disc surface widely spreads, precision of detectionsignals from the sensing portions is maintained. Further, as shown inFIG. 18B, the irradiation area of stray light from the disc surfaceoverlaps both of the upper right sensing portion and the lower rightsensing portion. However, in this case also, since the irradiation areaof stray light from the disc surface widely spreads as well as the caseshown in FIG. 18A, precision of detection signals from the sensingportions is maintained.

In the case where stray light is shifted in a left direction resultingfrom a lens shift, as shown in FIG. 18C, the irradiation area of straylight from the recording layer L3 overlaps the lower right sensingportion of the upper-side sensing portions. However, in this case, sincethe irradiation area of stray light from the recording layer L3 overlapsonly the lower right sensing portion, the irradiation area of straylight which may overlap a sensing portion can be reduced, as comparedwith the case where the irradiation area of stray light from therecording layer L3 shown in FIG. 17A is shifted in a left direction.Further, as shown in FIG. 18D, although the upper-side irradiation areaof stray light from the recording layer L3 overlaps the upper leftsensing portion of the right-side sensing portions, the lower-sideirradiation area of stray light from the recording layer L3 does notoverlap any of the sensing portions of the right-side sensing portions.Since stray light from the disc surface widely spreads in both of thecases shown in FIGS. 18C and 18D, precision of detection signals fromthe sensing portions is maintained, as well as the cases shown in FIGS.18A, 18B.

In the case where stray light is shifted in a right direction resultingfrom a lens shift, as shown in FIGS. 18E, 18F, the irradiation area ofstray light which may overlap the sensing portions is small, as well asthe cases shown in FIGS. 18C, 18D. In this case, as shown in FIG. 18E,the irradiation area of stray light from the recording layer L1 overlapsa lower right sensing portion of the upper-side sensing portions.

FIG. 20A is a diagram showing a simulation result on the relationsbetween a lens shift amount and a ratio of stray light entered intosensing portions, in the case where the light separating element Hs2 isused. FIG. 20A clearly shows that the ratio of stray light entered intothe sensing portions is small, as compared with the case shown in FIG.17C. In other words, use of the light separating element Hs2 is moreadvantageous in reducing the ratio of stray light which may enter intothe sensing portions, as compared with the case of using the lightseparating element Hs1.

FIGS. 19A through 19F are diagrams showing a simulation result in thecase where the light separating element Hs3 is used. FIGS. 19A through19F show distribution states of signal light and stray light near theupper-side sensing portions and the right-side sensing portions withrespect to the center O. In this simulation, BD light is also focused onthe recording layer L2. In FIGS. 19A through 19F, the reference sign“L2” denotes reflected light (signal light) from the recording layer L2,the reference signs “L1”, “L3” respectively denote reflected light(stray light) from the recording layers L1, L3, and the term “surface”denotes reflected light from the disc surface (light incident surface).

In the case where there is no lens shift, as shown in FIGS. 19A, 19B,the irradiation area of stray light from the disc surface does notoverlap the sensing portions, unlike the cases shown in FIGS. 18A, 18B.

In the case where stray light is shifted in a left direction resultingfrom a lens shift, as shown in FIGS. 19C, 19D, the area of an overlappedportion between the irradiation area of stray light from the discsurface and the sensing portions is small, as compared with the casesshown in FIGS. 18C, 19D. Further, as shown in FIG. 19D, the area of anoverlapped portion between the upper-side irradiation area of straylight from the recording layer L3 and the right-side sensing portions issmall, as compared with the case shown in FIG. 18D.

In the case where the irradiation area of stray light resulting from alens shift is shifted in a right direction, as shown in FIGS. 19E, 19F,the area of an overlapped portion between the irradiation area of straylight from the disc surface and the sensing portions is small, ascompared with the cases shown in FIGS. 18E, 18F. Further, as shown inFIG. 19E, the irradiation area of stray light from the recording layerL1 does not overlap the upper-side sensing portions, unlike the caseshown in FIG. 18E.

FIG. 20B is a diagram showing a simulation result on the relationsbetween a lens shift amount and a ratio of stray light entered intosensing portions, in the case where the light separating element Hs3 isused. FIG. 20B clearly shows that the ratio of stray light entered intothe sensing portions is small, as compared with the case shown in FIG.20A. In other words, use of the light separating element Hs3 is moreadvantageous in reducing the ratio of stray light which may enter intothe sensing portions, as compared with the case of using the lightseparating element Hs2.

The example of the invention has been described as above. The inventionis not limited to the foregoing example, and the example of theinvention may be modified in various ways other than the above.

For instance, in the foregoing example, the light separating element H1is disposed at a position anterior to the anamorphic lens 115.Alternatively, the light separating element H1 may be disposed at aposition posterior to the anamorphic lens 115. Further alternatively, adiffraction pattern for imparting substantially the same diffractionfunction as the light separating element H1 to laser light may beintegrally formed on the light incident surface or the light outputsurface of the anamorphic lens 115.

It is desirable to dispose the light separating element H1 at a positionanterior to the anamorphic lens 115, rather than disposing the lightseparating element H1 at a position posterior to the anamorphic lens115. Specifically, disposing the light separating element H1 at aposition anterior to the anamorphic lens 115 makes it possible tolengthen the distance from the light separating element H1 to thephotodetector 116, as compared with the case where the light separatingelement H1 is disposed at a position posterior to the anamorphic lens115. With this arrangement, as shown in FIG. 11, it is possible toirradiate plus first order diffraction light of BD light (signal light),on the photodetector 116, at a position sufficiently away from thecenter O, without the need of setting the diffraction angle of the lightseparating element H1 to a large value.

Further, in the foregoing example, as shown in FIG. 11, the four-dividedsensors C1 through C3 are disposed in up and down directions foracquiring a tracking error signal for CD based on an in-line method. Inthe case where the four-divided sensors C1 through C3 are disposed inthe manner as described above, the direction of a vector to be given bythe diffraction area H15 may be changed in up and down directions sothat plus first order diffraction light and minus first orderdiffraction light of BD light diffracted by the diffraction area H15 areirradiated onto the four-divided sensors C2, C3.

FIG. 21A is a plan view showing a light separating element H1 configuredin such a manner that the direction of the vector to be given by thediffraction area H15 is changed in up and down directions. In thisarrangement, the diffraction area H15 imparts vectors V45, V45 m to plusfirst order diffraction light and to minus first order diffraction lightof BD light passing through the light flux area a15. The directions ofthe vectors V45, V45 m are in parallel to the direction of a trackimage, and the magnitudes of the vectors V45, V45 m are equal to eachother.

In this arrangement, the magnitudes of the vectors V45, V45 m areadjusted in such a manner that the irradiation area for the four-dividedsensor Bz shown in FIG. 12, and a irradiation area located symmetricalto the aforementioned irradiation area with respect to the center O arerespectively located at the four-divided sensors C2, C3. In thisarrangement, the four-divided sensor Bz shown in FIG. 12 is omitted.

FIG. 21B is a diagram showing a distribution state of BD light (signallight) near the center O of the photodetector 116 in the abovearrangement. The four-divided sensors C2, C3 are disposed at positionssymmetrical to each other with respect to the four-divided sensor C1 inthe same manner as in the foregoing example.

As shown in FIG. 21B, zero-th order diffraction light of BD light(signal light) to be entered into all the diffraction areas H11 throughH15 is irradiated at the center O. Plus first order diffraction light ofBD light (signal light) diffracted by the diffraction area H15 isirradiated onto a central part of the four-divided sensor C2. Minusfirst order diffraction light of BD light (signal light) diffracted bythe diffraction area H15 is irradiated onto a central part of thefour-divided sensor C3.

In the above arrangement, assuming that detection signals from thesensing portions C21 through C24, C31 through C34 are respectivelyexpressed as C21 through C24, C31 through C34, HOEz, HOEθ can beacquired by the following equations (12), (13), in place of HOEz, HOEθto be used in the Z-adjustment and the θ-adjustment of the lightseparating element H1 expressed by the equations (10), (11).HOEz={{(C21+C24)−(C22+C23)}+{(C32+C33)−(C31+C34)}}/{(C21+C22+C23+C24)+(C31+C32+C33+C34)}  (12)HOEθ={{(C21+C22)−(C23+C24)}+{(C33+C34)−(C31+C32)}}/{(C21+C22+C23+C24)+(C31+C32+C33+C34)}  (13)

The light separating element H1 is positioned at such a position thatthe value of HOEz expressed by the equation (12) is set to zero inZ-axis direction, and is positioned at such a position that the value ofHOEθ expressed by the equation (13) is set to zero in a rotatingdirection with respect to the center O. Specifically, the adjustment ofStep S20 in FIG. 14 is performed based on the equations (12), (13). Byperforming the above operation, it is possible to properly set theposition of the light separating element H1 in Z-axis direction and in arotating direction with respect to the center O.

Further, in the foregoing example, there is exemplified an opticalpickup device compatible with BD, CD and DVD. Alternatively, theinvention may be applied to an optical pickup device compatible with BDand DVD, an optical pickup device compatible only with BD, or the like.For instance, in the case where the invention is applied to an opticalpickup device compatible only with BD, the optical systems for CD andDVD are omitted from the optical system shown in FIGS. 9A, 9B. In theabove modification, the four-divided sensors C2, C3, D1 through D3 areomitted from the sensor layout shown in FIG. 11.

Further, in the foregoing example and modification examples, thediffraction areas H15, H37 are disposed at the center of the lightseparating elements H1, H3. Alternatively, as shown in FIGS. 22A, 22B,the diffraction areas H15, H37 may be omitted. Further alternatively,the diffraction areas H15, H37 may be replaced by light blocking areas.In these modifications, the four-divided sensor Bz is omitted from thesensor layout shown in FIG. 11.

Furthermore, in the foregoing example, each of the diffraction areas H13and H14 of the light separating element H1 is divided into thediffraction areas H33, H34, and the diffraction areas H35, H36 shown inFIG. 15A; and the diffraction direction of each of the diffraction areasobtained by the division is adjusted in a slightly downward direction orin a slightly upward direction. Similarly to the above, each of thediffraction areas H31 and H32 shown in FIG. 15A may be divided into anupper portion and a lower portion, and the diffraction direction of eachof the diffraction areas obtained by the division may be adjusted in aslightly rightward direction or in a slightly leftward direction forsuppressing incidence of signal light onto the borderline between thesensing portions Ba1 and Ba4, and the borderline between the sensingportions Ba2 and Ba3.

The invention is preferably applied to an arrangement that a lightseparating element has a step-type diffraction pattern, as described inthe foregoing example. Alternatively, the invention may also be appliedto an arrangement that a light separating element has a blazeddiffraction pattern. Specifically, the invention may also be applied toa case where only one of plus first order diffraction light and minusfirst order diffraction light is generated, in addition to the casewhere both of plus first order diffraction light and minus first orderdiffraction light are generated.

Furthermore, the diffraction directions of laser light by the lightseparating element are not limited to the ones described in theforegoing example. As far as it is possible to allow laser light in twolight flux areas in the direction of one pair of vertically oppositeangles, and laser light in other two light flux areas in the directionof another pair of vertically opposite angles to be entered at positionsaway from each other, on the light receiving surface of thephotodetector, when an intersection of two straight lines respectivelyextending in parallel to the flat surface direction and the curvedsurface direction and intersecting with each other is aligned with theoptical axis of laser light, the diffraction directions of laser lightby the light separating element may be set in directions other than thedirections shown in the foregoing example.

The embodiment of the invention may be changed or modified in variousways as necessary, as far as such changes and modifications do notdepart from the scope of the claims of the invention hereinafterdefined.

1. An optical pickup device, comprising: a laser light source; anobjective lens which focuses laser light emitted from the laser lightsource on a recording medium; an astigmatism element into which thelaser light reflected on the recording medium is entered, and whichconverges the laser light in a first direction for generating a firstfocal line and converges the laser light in a second directionperpendicular to the first direction for generating a second focal line;a photodetector which receives the laser light passing through theastigmatism element; and a light separating element into which the laserlight reflected on the recording medium is entered, and which guides thelaser light entered into two first areas and into two second areas tofour positions different from each other, on a light receiving surfaceof the photodetector, wherein the photodetector has a plurality ofsensing portions disposed at the four different positions, when anintersection of two straight lines respectively extending in parallel tothe first direction and the second direction and intersecting with eachother is aligned with an optical axis of the laser light, the two firstareas are disposed in a direction along which one pair of verticallyopposite angles defined by the two straight lines are aligned, and thetwo second areas are disposed in a direction along which another pair ofvertically opposite angles are aligned, and the light separating elementimparts a light separating function to the laser light entered into thetwo first areas in directions identical to each other and withmagnitudes different from each other, and imparts a light separatingfunction to the laser light entered into the two second areas indirections identical to each other and with magnitudes different fromeach other.
 2. The optical pickup device according to claim 1, whereinthe light separating element is operable to guide the laser light to beentered into the two first areas to vertex angular positionscorresponding to diagonal positions of a first rectangle, on a lightreceiving surface of the photodetector, and is operable to guide thelaser light to be entered into the two second areas to vertex angularpositions corresponding to diagonal positions of a second rectangledifferent from the first rectangle, on the light receiving surface ofthe photodetector.
 3. The optical pickup device according to claim 1,wherein the light separating element has a step-type diffraction patternoperable to separate the laser light by diffraction, and thephotodetector has a four-divided sensor which receives the laser lighttransmitted through the light separating element without beingdiffracted on the diffraction pattern.
 4. The optical pickup deviceaccording to claim 1, wherein the light separating element furtherincludes a third area for suppressing laser light entered into a centralpart of the light separating element from being guided to the fourdifferent positions.
 5. The optical pickup device according to claim 1,wherein the light separating element is disposed at such a position thatthe second area is arranged in a direction of a track image, and thesecond area is divided into two portions by a straight line passingthrough a center of the light separating element and extending inparallel to the direction of the track image, and a light separatingfunction of the two portions of the second area is adjusted in such amanner that a predetermined clearance is formed between irradiationareas, on the photodetector, of the laser light entered into the twoportions of the second area.
 6. The optical pickup device according toclaim 3, further comprising: another laser light source which emitslaser light of a wavelength different from a wavelength of the laserlight to be emitted from the laser light source; a diffraction gratingwhich divides the laser light emitted from the another laser lightsource into a main beam and two sub beams; another objective lens whichfocuses the main beam and the two sub beams on another recording medium;and an optical system which is configured to allow the main beam and thetwo sub beams reflected on the another recording medium to enter intothe astigmatism element and into the light separating element, and guidethe main beam out of the main beam and the two sub beams transmittedthrough the light separating element without being diffracted on thediffraction pattern, to the four-divided sensor, wherein thephotodetector has another two four-divided sensors which receive the tworespective sub beams transmitted through the light separating elementwithout being diffracted on the diffraction pattern.